In the course of development of pollution-free driving systems for motor vehicles, various systems have been proposed for operating vehicles by means of piston engines driven by compressed air. These systems usually involve the use of a source of compressed air, such as a pressure tank, from which the working fluid is supplied to the engine cyclinders through suitable admission valves suitably timed to provide power strokes of the pistons, and on the return strokes the expanded air is allowed to discharge from the cylinders via suitably timed exhaust valves. The air is filtered by using a hydrovane compressor which has intake filters that remove pollution, making the exhausted air more purified than it was when it entered the compressor. This hydrovane system is a constant flow air compressor. It will put out 25 pounds pressure per minute up to 200 pounds per minute.